The present invention relates to an optical scanner and more particularly to an optical scanner capable of generating and utilizing multiple, simultaneous scan lines which are focussed at different points in space.
Optical bar code scanners are widely used in different industries for different purposes. Such scanners are commonly used with processor-controlled point of sale systems in retail stores and supermarkets to identify goods being processed. A bar code label on an item provides input data which can be used by the system to retrieve price data, print customer receipts and update inventory records. There are many less well known applications for optical bar code scanners, such as the use of such scanners to track major components of complex products during assembly operations.
There are basically two types of optical bar code scanner systems. The first type is a hand-held system in which an operator manually moves an optical detector or wand across a bar coded label to be scanned. Signals generated by the wand are transmitted, usually via a cable, to a controller or terminal where the label information can be processed.
The second type of scanner system is a fixed head scanner. In a fixed head scanner system, an item to be detected is moved across a transparent scan window, normally in the upper surface of a stationary scanner. The scanner may, for example, be built into the checkout stand in a supermarket. Optical elements within the scanner deflect a laser beam to produce scan lines which sweep through a volume above the scan window. Light reflected from an object passing through the volume is detected. Potential label candidates are selected and decoded by electronic circuits or microprocessors within the scanner. The present invention relates to this fixed head type of scanner.
A scanner should be able to detect most labels on the first pass of each labeled product over the scanner window. If a label on a product is not successfully read, that product must be brought back across the scanner window in one or more additional passes. The additional passes or re-scans are obviously time consuming and thus harmful to productivity.
The reading performance of a scanner is affected by a number of different factors. Factors such as the print quality of the labels being scanned, the electrical characteristics of the signal detecting system and the decoding algorithms which are employed, all have an impact on reading performance.
Another factor which influences reading performance is the scan pattern generated by the scanner. To increase the chances that a randomly-oriented bar coded label will be scanned completely during a pass over the scanner window, multi-line scan patterns are used in which the lines intersect each other at angles. Such scan patterns are generated by directing a laser beam at a rotating mirror wheel or through a rotating holographic disc to cause the laser beam to be deflected in predetermined directions. Fixed arrangements of mirrors are used to change the beam paths before the beams exit from the scan window to form scan lines. Known scan patterns include non-parallel or intersecting lines and/or parallel lines displaced in space from one another.
To improve the chances that a label will be properly scanned, it is desirable to increase the number of individual scan lines in the scan pattern. With prior art systems, there is a point of diminishing returns. In such systems, the number of scan lines can be increased only by making each of the scan lines shorter. Shorter scan lines are less effective so that a pattern with many short scan lines may be no more effective overall than a pattern with fewer, longer scan lines.
Another technique for improving reading performance of a scanner is to focus the individual scan lines at different points in space relative to the scan window. Some of the scan lines will be more effective in detecting labels while those labels are still some distance from the scan window. Other scan lines will be most effective in detecting labels immediately above the scan window.
While the use of multiple focal planes can improve reading performance, the degree of improvement is still limited by the fact that only one scan line is generated at any given time.